I want to use

Event OnUpdateGameTime()

to toggle some parent objects on/off according to the day/night cycle. How much stress would be placed on the game engine if I used

RegisterForSingleUpdateGameTime(12)

to poll the game time every 12 game hours?

It shouldn't cause much overhead. The script will be dormant that whole time so it won't be taking up any resources. However, you may want to keep in mind that simply updating every 12 hours with that method may not work as intended, since OnUpdateGameTime() will be delayed until after the player sleeps/waits/etc. It may throw off your timing cycle if that matters to you.